A conventional watermark, on a paper document, may consist of a translucent design which is visible when the document is held to the light. Or, more generally, a watermark may be viewed under certain lighting conditions or at certain viewing angles. Such watermarks, which are difficult to forge, can be included for the sake of authentication of documents such as bank notes, checks and stock certificates, for example.
In digital video technology, watermarks are being used to betoken certain proprietary rights such as a copyright, for example. Here, the watermark is a visible or invisible pattern which is superposed on an image, and which is not readily removable without leaving evidence of tampering. Resistance to tampering is called “robustness”.
One robust way of including a visible watermark in a digitized image is described by Braudaway et al., “Protecting Publically Available Images with a Visible Image Watermark”, IBM Research Division, T. J. Watson Research Center, Technical Report 96A000248. A luminance level, ΔL, is selected for the strength of the watermark, and the luminance of each individual pixel of the image is modified by ΔL and a nonlinear function. For increased security, the level ΔL is randomized over all the pixels in the image.